Log Message:
devel/ruby-tty-prompt: update to 0.21.0

Update ruby-tty-prompt to 0.21.0.

## [v0.21.0] - 2019-03-08

### Added
* Add :min option to #multi_select prompt by Katelyn Schiesser(@slowbro)

### Changed
* Change gemspec to remove test artifacts

### Fixed
* Fix :help_color option for multi_selct prompt by @robbystk